1. Field of the Invention
The present invention relates, for example, to an ion beam etching method, an ion beam etching apparatus, a recording medium, and a method of manufacturing a semiconductor element, and, more particularly, to an ion beam etching method, an ion beam etching apparatus, and a recording medium which can prevent the occurrence of ESD.
2. Description of the Related Art
In an ion beam etching process, a substrate arranged on a substrate holder is irradiated with positive ions (ion beam) and etched. An ion beam etching apparatus includes an ion source which extracts positive ions from a plasma using an extraction electrode, and a substrate holder which is placed at the position where it squarely faces the ion source, and holds a substrate in a vacuum. In general, the ion beam etching apparatus also includes an electron source (neutralizer) which emits electrons to neutralize the positive ions extracted from the ion source, a shutter device which is placed between the ion source and the substrate holder, and shields the ion beam, and an exhaust device which evacuates a process chamber into a vacuum (see, for example, Japanese Patent Laid-Open No. 2002-353172).
The substrate is etched by the extracted ion beam. At this time, the positively-charged ion beam may generate a positive potential in the substrate. This positive potential may damage an insulating film formed on the substrate. Such damage is commonly called electrostatic damage (ESD). Especially elements including insulating films having line widths of several nanometers, such as a tunneling magnetoresistance (TMR) element, are more likely to suffer electrostatic damage, so it is desirable to suppress a fluctuation in substrate potential in a process of fabricating these elements.
A fluctuation in substrate potential during an etching process is suppressed by emitting electrons which neutralize a positive potential toward the gap between the extraction electrode and the substrate. The electrons are emitted by the neutralizer arranged near the extraction electrode of the ion source. The amount of emitted electrons is adjusted so that the substrate potential has a desired value. The start and end of the etching process is controlled by opening/closing the shutter. During the shutter operation, the amount of ion beam incident on the substrate, and the amount of electrons emitted by the neutralizer fluctuate depending on the opening degree of the shutter. At this time, if the balance between the amount of ion beam incident on the substrate and the amount of electrons emitted by the neutralizer degrades, the substrate potential fluctuates.
FIG. 5 shows the measurement result of the substrate potential (monitor voltage 21) when the shutter opens while the amount of electrons (neutralizing current 21a) emitted by the neutralizer is kept constant, for the sake of descriptive convenience. Referring to FIG. 5, point A indicates the timing at which the shutter starts its opening operation, and point D indicates the timing at which the shutter completes its opening operation. As can be seen from FIG. 5, the substrate potential 21 considerably fluctuates during the shutter operation. Therefore, in a known technique, a fluctuation in substrate potential, which depends on the shutter operation state, is suppressed (see, for example, Japanese Patent Laid-Open Nos. 2002-353172 and 2002-075968).
Japanese Patent Laid-Open No. 2002-353172 discloses a technique of suppressing a fluctuation in substrate potential by controlling the output from the ion source during the shutter opening operation. More specifically, to overcome insufficient neutralization of the substrate potential while opening the shutter, this patent literature describes decremental control of the amount of emitted ion beam or the acceleration voltage of the ion beam while opening the shutter.
Japanese Patent Laid-Open No. 2002-075968 discloses a technique of applying a reverse voltage to a shutter plate to prevent electrons from flowing into the shutter. That is, this patent literature describes a technique of preventing electrons from flowing into the shutter to reduce insufficient neutralization encountered while closing the shutter.
Unfortunately, an apparatus adopted in Japanese Patent Laid-Open No. 2002-353172 has a configuration in which the shutter device can completely isolate the space on the substrate side from the ion beam and electron sources, so the ion beam and the electrons (that is, the plasma) are confined in a tightly-sealed, narrow space between the ion source and the shutter while the shutter is closed. Therefore, the same effect cannot be obtained in a shutter device which does not completely isolate the space on the substrate side and the space in which a plasma is generated, that is, a shutter device which allows the plasma to enter the space on the substrate side even while the shutter is closed.
Also, the technique disclosed in Japanese Patent Laid-Open No. 2002-353172 fluctuates the ion beam amount, so the amount of etching of the substrate may considerably fluctuate during the shutter opening/closing operation, and the quality of an element formed on the substrate may, in turn, fluctuate. Furthermore, the technique disclosed in Japanese Patent Laid-Open No. 2002-075968 applies a voltage to the shutter plate, but can hardly be applied to a shutter device that allows the plasma to enter the space on the substrate side even while the shutter is closed.